1. Field of the Invention
The invention relates to an improved process for the preparation of dialkyl carbonates.
Dialkyl carbonates, in particular dimethyl carbonate, are intermediate products which have a low toxicity, and can replace toxic intermediate products, such as phosgene or dimethyl sulphate, in many reactions. It is furthermore non-corrosive. No environmentally harmful by-products are formed when it is used.
Examples of such reactions of dialkyl carbonates are the preparation of urethanes from aliphatic or aromatic amines, which in turn can be split to give the corresponding isocyanates. Dimethyl carbonate, for example, can also replace dimethyl sulphate in the quaternisation of amines or in the methylation of phenol or of naphthols. Dimethyl carbonate furthermore can be added to petrol for cars as an agent which improves the octane rating, for example instead of lead compounds. In view of this importance of dialkyl carbonates, there is still a lack of an industrially simple and environmentally compatible production process which is suitable for large capacities without substantial formation of by-products or linked substance circulations.
2. Description of the Related Art
There are various preparation processes for the preparation of dialkyl carbonates which have also already been tried industrially on a small scale. The preparation routes based on the catalytic reaction of alkanols with carbon monoxide and oxygen in accordance with the following equation have been worked on intensively by various study groups: ##STR1## The copper compounds which act as the catalyst have thus been employed in the form of various copper salts. When copper(II) chloride is used as the catalyst in accordance with JP-45/11129 (1970), unsatisfactory selectivities are achieved. Trouble is caused above all by the formation of relatively large amounts of methyl chloride, which, because of its great volatility, tends to spread ubiquitously in the entire production plant and can cause corrosion practically in the entire plant.
A better selectivity is obtained when organic complexing agents are used (DE-A 2,110,194), but here there is the problem of separating off the catalyst salts, which are partly dissolved in the reaction mixture but to a greater extent are present as a suspension.
Carrying out this reaction in accordance with DE-A 2,743,690 especially presents problems, since the catalyst salts are practically completely undissolved in the reaction mixture but merely suspended. These salts must be conveyed through the reaction zone and through the cooling units, and separated off mechanically, for example with the aid of centrifuges, after the reaction. In addition to the corrosion already mentioned, this also causes erosion, poor heat transfer as well as blockages and encrustations.
In order to avoid these disadvantages of a catalyst circulation, it has been proposed to keep the catalyst salts suspended in stationary form in the reactor and to meter methanol, CO and oxygen into the reactor, while the dialkyl carbonate formed and the water of reaction are distilled off from the reactor, together with the methanol employed in excess (EP 0,413,215 A2). The liquid reaction medium here essentially comprises the alkanol to be reacted (EP-0,413,215, page 3, line 52), so that the molar ratio between the alkanol and the Cu salt is very high (preferably 1:0.01-0.05). This has the disadvantage that the rate of reaction is relatively low. The need to establish a low dialkyl carbonate concentration also presents problems here. This is not easy, since the reaction is carried out under a high system pressure, and the solubilities of the dialkyl carbonate and also of the water in the reaction medium, which essentially comprises methanol, are very high. This means that removal of the dialkyl carbonate and water must be forced with a relatively large amount of inert gas or methanol gas.
Moreover, with this proposal it is to be expected that after some time the catalyst must be exchanged or a certain proportion of it must be constantly renewed, which is associated with the abovementioned problem of removal, regeneration and recycling of the catalyst.